This invention relates to rechargeable lithium ion electrochemical cells.
A battery includes one or more galvanic cells (i.e., cells that produce a direct current of electricity) in a finished package. In each cell, two electrodes are separated by an electron insulator, but are joined by an ion-carrying path. The electron-carrying path of the battery is external; the path proceeds, via a conductor, through a device where work is done. The ion-carrying path of the battery is internal and proceeds via an electrolyte.
The electrodes are usually composed of dissimilar metals. The electrode where an electrolytic species receives electrons is the positive electrode, also referred to as the cathode. The electrode where an electrolytic species goes into solution, releasing electrons, is called the negative electrode, or anode. The electrolyte generally is composed mainly of an ionizable salt dissolved in a solvent.
Batteries may be rechargeable; such batteries are called "storage" or "secondary" batteries. Storage batteries can be recharged by passing current through the cells in the opposite direction of discharge current flow. The charging current restores the chemical conditions of the battery, preparing it to be discharged again. Primary batteries, on the other hand, are meant to be discharged to exhaustion once, and then discarded.
An example of a rechargeable battery is a lithium ion cell. The positive electrode of this cell can include, for example, a lithiated metal oxide, such as LiCoO.sub.2, LiNiO.sub.2, or LiMn.sub.2 O.sub.4. The negative electrode can be, for example, a carbon or metal oxide electrode. The electrolyte in lithium ion cells can include a lithium salt (e.g., LiPF.sub.6 or LiClO.sub.4) dissolved in an aprotic solvent such as, for example, propylene carbonate or ethylene carbonate. An electrode separator is located between the positive electrode and negative electrode to prevent physical and electrical contact between the electrodes. Physical contact between the positive electrode and negative electrode leads to short circuiting which discharges the cell. The electrode separator insulates the electrodes from contact. The separator is porous (e.g., a porous organic polymer) and allows the electrolyte to migrate from one electrode to the other.
An example of the electrochemical process in a lithium-ion battery is as follows: ##STR1## where A.sub.2 B.sub.w represents the negative electrode (e.g., carbon), Li.sub.y M.sub.n Y.sub.m represents the positive electrode (e.g., LiCoO.sub.2), Li(.sub.y-x)M.sub.n Y.sub.m represents the lithium-depleted positive electrode (e.g., LiCoO.sub.2 /CoO.sub.2), and Li.sub.x A.sub.z B.sub.w represents the lithium-enriched negative electrode (e.g., Li.sub.x C.sub.6). The lithiated metal oxide provides the source of lithium ions which shuffle back and forth between anode and cathode when the cell is charged and discharged.
Generally, the charge (or discharge) capacity is the amount of charge the cell accepts (or provides) to reach to the charge (or discharge) voltage (e.g., of 4.1 V or 2.8 V). The difference between charge and discharge capacity is the irreversible charging capacity.
Lithium ion cells tend to exhibit a loss in charging capacity during the first few charge/discharge cycles, which is possibly due to consumption of lithium. The loss in charging capacity is irreversible, and leads to decreased charge capacities in the cells because the positive electrode is not fully lithiated.
A characteristic of the materials used for both positive and negative electrodes is their intrinsic partial irreversibility. When carbon is used as a negative electrode in a lithium ion cell, the first charging capacity is always significantly higher than the first discharge capacity. For common carbon negative electrode materials, the irreversibility ranges from 10% to 30% of the first charge capacity (i.e., the first discharge capacity is only 90% to 70% of the first charge capacity). For typical positive electrode materials, however, the irreversibility is generally much lower. For LiCoO.sub.2, for example, the first charge/discharge irreversibility is less than 5%. That is, the first discharge capacity is 95% of the first charge capacity.
Consequently, for design efficiency, the electrodes in a rechargeable cell generally are balanced (i.e., each electrode should have the same capacity). When positive and negative electrodes of different irreversibility are used together in a rechargeable cell, the cell typically is balanced in a manner that the first charge capacities of the two electrodes are the same. On use, the cell will only cycle to the reversible capacity of the most irreversible electrode. Therefore, the full reversible portion of the other electrode is not fully utilized. This can result in a lower than maximum capacity of the completed electrochemical cell.